Friedreich ataxia (FRDA; OMIM 229300) is an autosomal recessive neurodegenerative disease caused by reduced levels of the mitochondrial protein frataxin (FXN), which is due to large expansions of a repetitive GAA sequence located in the first intron of the FXN gene. The expanded GAA repeat tract inhibits transcription of the FXN gene by inducing heterochromatinization, and results in reduced levels of FXN mRNA and protein in all cells of the body. The length of the GAA repeat tract and FXN levels inversely correlate with disease severity, and are considered quantifiable molecular hallmarks of FRDA. Characteristic physical symptoms of FRDA include discoordination, slurred speech, muscle weakness, peripheral neuropathy, and cardiomyopathy. At the present time there is no effective treatment for this debilitating and progressive disease. Additionally, efficacy studies of therapeutic approaches currently being developed are hampered by a lack of appropriate animal models that recapitulate the complexity of FRDA molecular and clinical phenotypes. Technical limitations pertaining to the instability of long GAA repeat sequences have thus far prevented the field from attaining a FRDA GAA repeat knock-in (KI) mouse model carrying tracts greater than 230 repeats, which is considerably shorter than the average pathogenic length observed in patients (600 ? 800 repeats). As expected, the current FRDA KI models with short repeats exhibit only mild molecular and behavioral phenotypes of FRDA, and thus are of limited use in pre-clinical studies of drug candidates. Herein, we provide preliminary evidence of our innovative strategy to overcome this technical obstacle and create an improved GAA repeat model bearing 750 GAA repeats in intron 1 of the mouse Fxn gene. Based on published evidence and our preliminary data, we hypothesize that a new FRDA KI mouse model carrying long tracts of ~750 GAA repeats in intron 1 of the Fxn gene will demonstrate a robust phenotype that resembles many aspects of FRDA and will be well-suited for testing the efficacy of therapeutic approaches for this disease. To create and validate our FRDA KI expanded GAA repeat mouse model, we propose the following Specific Aims: 1.) Development of a new FRDA KI mouse model carrying 750 GAA repeats. 2.) Evaluation of new homozygous FRDA KI-Fxn750/750 and hemizygous FRDA KI-Fxn750/- FRDA mouse models to select the most robust model for internal and external validation studies. 3.) Internal and external validation of FRDA KI mouse models carrying 750 GAA repeats. We expect that by engineering a mouse model that mimics the genetic mutation of the human FRDA condition, with expanded GAA tracts of 750 repeats, the molecular and behavioral phenotypes will be apparent and quantifiable. Generation and standardized internal and external validation of this ?second generation? FRDA KI mouse will allow for comprehensive and comparative evaluation of novel drug candidates in one model system that recapitulates many critical aspects of the disease phenotype and etiology.